a) Field of the Invention
The present invention relates to a hot wire air flow meter, said meter being capable of measuring the flow rate of air with a so-called hot wire made of, for example, platinum. In particular, this invention is applicable to a hot wire air flow meter capable of producing a pulsed output function and which is suitable for measurement of intake air flow rate of an internal combustion engine for a motor vehicle.
b) Description of the Prior Art
A block schematic diagram of a hot wire flow meter having a pulsed output function is shown in FIG. 1 in which a constant temperature control circuit 1 is exposed to air flow and the temperature control circuit operates so that the resistance of the hot wire is kept constant thereby maintaining the hot wire temperature constant. The circuit 1 output voltage v.sub.1 represents air flow rate Q, but this voltage v.sub.1 includes an offset voltage with regard to the flow rate Q, which must firstly be compensated. Thus the voltage v.sub.1 is applied to a voltage zeroing circuit to zero the v.sub.1 /Q characteristic and the thus compensated characteristic has the gradient (gain) thereof modified by an amplifier. These functions are performed by what is termed herein as a zero/span circuit 2, the output voltage v.sub.2 of which is converted into a pulsed frequency by voltage controlled oscillator 3 (herein below abbreviated to VCO) having a pulse voltage v.sub.3 with a frequency f proportional to the voltage value v.sub.2. A reference voltage circuit 4 provides a reference voltage V.sub.s to the circuits 1- 3.
FIG. 2 shows a specific example of the constant temperature control circuit 1, the zero/span circuit 2, and the reference voltage circuit 4. A specific example of the VCO circuit is shown in FIG. 3. These circuits are principally formed by discrete elements, although some parts are formed in an integrated circuit (IC), and the respective elements are usually attached to a hybrid substrate.
The constant temperature control circuit 1 in FIG. 2 has a hot wire 11, a cold wire 12, operational amplifiers 5, 6, a power transistor 10 having its collector connected to a supply voltage terminal 9, a bias resistor 20, a capacitor 22 for suppressing electromagnetic noise on the hot and cold wires and a bias resistor 21 for the operational amplifier 5. A smoothing capacitor 23 is connected across the input of operational amplifier 6. Even when the hot wire 11 is cooled by air, the circuit 1 operates to control the current flowing through the hot wire 11 so that the temperature thereof is always maintained constant.
The zero/span circuit 2 is mainly formed by an operational amplifier 7 and biasing resistors 70-73, and 77-79, and performs zeroing and gain compensation with regard to the signal voltage v.sub.1 derived from the interconnection of the hot wire 11 and resistor 20 by properly selecting the resistances 70-73, 77-79. A Zener diode 14 is used for surge protection and an analogue voltage signal v.sub.2 is output to terminal 30.
The reference voltage circuit 4 is formed by an operational amplifier 8, resistors such as 81, 83, 84, 85, Zener diodes 80, 13, a diode 82, and capacitor 15. By these elements the constant voltage between both ends of the Zener diode 80 is amplified and the constant voltage V.sub.s which is produced is supplied to the respective circuits 1-3.
The resistor 84, Zener diode 13, and capacitor 15 constitute a surge protection circuit.
The VCO 3 shown in FIG. 3 is formed by operational amplifiers 901 and 24, resistors such as resistors 32, 33, a capacitor 35, and a transistor 34. The amplifier 901 is connected to a bias voltage terminal 902. The circuit of the operational amplifier 901 and the capacitor 35 is configured to function as an integrator, and the circuit composed mainly of the operational amplifier 24, and the resistor 33 is arranged to function as a comparator. When the analog voltage v.sub.2 is applied to an input terminal 30, it is converted into a pulse output voltage v.sub.3 supplied to terminal 31 having a frequency proportional to the applied voltage v.sub.2.
Prior art air flow meters similar to the above are disclosed in, for example:
JP-A-59-224427 (1984) PA0 JP-A-60-178317 (1985) PA0 JP-A-61-1847 (1986) PA0 JP-A-61-17019 (1986) PA0 JP-A-61-104246 (1986) PA0 JP-A-62-79316 (1987).
In the above described prior art, optimization of the circuit construction was not specifically considered, therefore such problems as precision could not be ensured, temperature compensation is poor and electromagnetic wave immunity performance (EMI) is poor. In these respects, the precision in function of the operational amplifiers is principally determined by the ratios of the biasing and feedback resistors and when, for example, the biasing resistors are on one chip and the feedback resistor on another chip, the amplifier cannot provide proper control since the resistors are affected by different temperatures. With regard to EMI, this is caused by interference from, for example, mobile telephones which is picked up by the wires interconnecting chip circuits together.
One object of this invention is to provide a high precision hot wire air flow meter having improved resistance to temperature and electromagnetic wave effects.
It is an object of a feature of this invention to provide an integrator type filter circuit for use in a hot wire air flow meter which is better suited for manufacture on an integrated circuit.